Slide two edge loops away from each other
For two existing edgeloops the only way I know of is through scaling.
Scale the two edgeloops with S and constraint to the relevant axis with X, Y or Z.

Of course this only works with axis aligned loops, and with evenly sized geometry on both sides, otherwise it will create distortions.

If you don't mind creating new edgeloops then beveling does the trick.
Just create a new edge loop with Ctrl + R, then Right Click to leave it in place. Select the newly created edge loop and bevel it with Ctrl + B.

This works even on skewed or twisted geometry, non axis aligned loops, and has the added benefit of allowing absolute dimensions for distance, as opposed to only percentage factors with true edge sliding.